By considering the effect of intermolecular force, the apparent viscosity of non-polar-molecular liquid flowing in microchannels with isotropic solid wall is studied. The molecular theory of apparent viscosity in microchannel taking into account the effects of channel walls is proposed. Numerical results show that the effect of the wall on liquid apparent viscosity can be very strong. The intensity of effect is determined by the intermolecular force between the liquid and the wall. Meanwhile, the effect of the wall decreases rapidly with the increase of distance from the wall. For liquid argon, the extent of influence is within 10 layers of molecules, which is about 4 nm from the wall.